The present invention relates to gears, and more particularly, but not exclusively, relates to reduction of backlash in gear trains.
When the tooth of one gear mates with the gap of another gear, the gap typically provides more space than needed to accommodate the tooth. This excess space is sometimes called "lash" or "backlash." Backlash may vary with a number of factors including radial play in the gear bearings, gear shaft eccentricity, incorrect center-to-center spacing of the gears, and the gear-to-gear variation typical of many gear manufacturing processes.
The extra space associated with backlash usually leads to significant impact loading of the gear teeth. This loading often causes excessive noise and may result in other gear train problems. For example, backlash may accelerate gear wear. Backlash reduction is of particular concern for internal combustion engine applications--especially for gear trains used with diesel engines. U.S. Pat. No. 5,450,112 to Baker et al., U.S. Pat. No. 4,920,828 to Kameda et al., U.S. Pat. No. 4,700,582 to Bessette, and U.S. Pat. No. 3,523,003 to Hambric are cited as sources of background information concerning the application of gear trains to various engines.
One way to reduce backlash is through precision machining and mounting of the gears. However, this approach is usually expensive and still may not adequately address backlash that changes over time due to wear. Another approach to reduce backlash has been the introduction of one or more scissor gears into the gear train. Generally, scissor gears have teeth which adjust in size to occupy the space available between teeth of a mating gear. U.S. Pat. No. 5,056,613 to Porter et al., U.S. Pat. No. 4,747,321 to Hannel, U.S. Pat. No. 4,739,670 to Tomita et al., U.S. Pat. No. 3,365,973 to Henden, and U.S. Pat. No. 2,607,238 English et al. are cited as examples of various types of scissor gears.
Backlash accommodation with a scissor gear is often limited when the scissor gear is meshed with two or more gears having different amounts of lash. Typically, the mating gear having the smallest amount of lash dictates the effective tooth size of the scissor gear; however, this size is generally inadequate to take-up the greater lash of the other mating gear or gears. One potential solution to this problem is to select mating gears which minimize the lash difference, but this "lash matching" approach is typically expensive and time-consuming. Consequently, a need remains for a gear train assembly which accommodates lash differences resulting from multiple gears meshing with a scissor gear.
One scissor gear configuration has two toothed wheels spring-biased to rotate relative to each other about a common center. For this configuration, paired gear teeth, one from each wheel, spread to occupy the available space between teeth in a mating gear. In some gear trains, loading of the tooth pairs by the mating gear becomes high enough to align each tooth pair in opposition to the spring bias. Typically, each member of the aligned pair is configured to proportionally bear this high load by being sized with the same nominal thickness. However, it has been found that random deviations from nominal are usually enough to cause one tooth or the other of each pair to bear a disproportionately high amount of the load until it has deformed enough to match the other tooth. This deformation process often subjects the gear teeth to reverse bending loads that more quickly wear-out the teeth compared to teeth subjected to unidirectional bending loads. Also, such deformation may cause greater tooth-to-tooth variation, resulting in poorer performance and a more noisy gear train. Therefore, a need exists for an anti-lash gear assembly which accommodates high loading without these drawbacks.
It has also been discovered that the knocking of heavy duty diesel engines, often attributed to combustion processes, results, at least in part, from high impact gear tooth noise. Typically, this noise is not sufficiently abated by conventional scissor gear configurations. Thus, a gear train is also in demand which addresses this type of noise.